Speedometer acting up on Volvo 2015

Hi everyone!
I started having an issue a couple of days ago.
My speedometer while driving it’s starts at point showing that I’m going faster then I am … the arrow is oscillating back and forward from the actual speed I’m driving to like 20-35 miles over and back the speed I’m driving … and sometimes the needle just drops down to zero … and at times it works normal .
Anyone had this problem or know what the problem can be ?
I’ll mention also that a while ago I added a with which I can cut the speedometer sensor off by a push of a button ( for the yard moves and small stuff like that )
Also adding that my Volvo is automatic.
Thank you !